This invention relates to composite manufacturing, and, more specifically, to consolidating three dimensional, multi-ply preforms to net mass and final shape with reduced distortion.
Matched-die compression molding is commonly used in fabricating advanced composites when high dimensional precision and surface strength requirements must be met. However, it can take much more effort to develop a compression molding process than an autoclave process. In compression molding, optimization of preform geometry, thermal cycles, and mold closure is critical to quality of the end product, and control of these process parameters must be very precise.
Geometric features such as thick sections, large variation in cross-sectional thickness or high ply drop-off, and high curvatures in composite parts are known to be difficult to process. In general, the interior and exterior of the material due to low thermal through-thickness conductivity of the composite, are in different cure and viscosity states at any particular point in time. For parts with variable sectional thickness, a thicker section contacts the mold earlier than a thinner one due to the difference in bulk, which further exacerbates the differential pressure during the molding phase.
Fiber wrinkling or buckling, resulting from creeping of the material when under pressure, is an important problem in manufacturing fiber-reinforced composites because such defects can lead to degradation of mechanical performance. This degradation is particularly important in parts such as gas turbine engine composite fan blades which rotate and develop substantial centrifugal loads which must be carried by the structural composite plies.
A typical fan blade includes an airfoil extending radially from root to tip and axially from leading edge to trailing edge. The airfoil root is integrally formed with a suitable dovetail which is used for mounting the individual fan blades to the perimeter of a rotor disk. The airfoil typically twists along its stacking axis from root to tip and has varying curvature or camber therealong. The airfoil increases in thickness from the leading and trailing edges to the mid-chord regions thereof, and also increases in thickness from the tip to the root. At the root, the airfoil transitions into the dovetail which is substantially thicker for carrying the significant centrifugal loads into the rotor disk during operation.
An exemplary composite fan blade may have several hundred composite plies defining the root, and tapers down to a few hundred composite plies at the inner span of the airfoil. The number of plies further decreases from the airfoil root to its tip down to about one hundred plies thereat.
Each composite ply typically includes a weave or cloth of suitable structural fibers, such as glass or graphite fibers, in a suitable resin matrix. The several plies are individually configured so that when stacked together they collectively define a preform having generally the shape of the resulting fan blade. The preform must be suitably molded to final shape and cured to form the resulting fan blade.
As indicated above, a pair of matching dies may be used to compression mold the preform to final shape. Or, an autoclave process may be used wherein the preform is positioned atop a single mold, with a uniformly flexible caul positioned atop the preform for providing a surface against the pressurized gas used to conform the preform to the mold under heat and forming the fan blade.
In both processes, consolidation of the preform is required during which the thickness of the preform is reduced under pressure and temperature with corresponding cross linking and curing of the matrix to form the final configuration of the part or blade.
During the molding process, the preform inherently undergoes plastic deformation as it is molded to shape. In view of the varying thickness of the exemplary fan blade and its complex three-dimensional configuration, the amount of thickness compression and plastic deformation of the preform correspondingly varies. For example, the dovetail is relatively thick and uniform and transitions sharply to a narrower neck region at the root of the airfoil. In this region, the number of plies in the preform decreases substantially on the order of a several hundred ply decrease. The amount of ply variation along the remainder of the airfoil to its tip is relatively small in comparison, also with relatively small transition in thickness throughout the relatively thin airfoil.
Accordingly, under pressure and temperature, consolidation of the preform varies with typically more consolidation at the thicker dovetail and less consolidation at the airfoil tip. In the matched-die compression molding process described above, die travel is necessarily uniform over the entire surface area of the preform, with the thicker, dovetail portion contacting the mold earlier than the thinner tip portion. This is necessary to ensure effective consolidation at the root without undesirable overcompression at the tip which would misshape the final blade. As a result of this process, the preform is susceptible to undesirable wrinkling or buckling, especially in regions of the blades having large thickness variation such as at the root. Porosity, delamination, and other defects are also possible during the process.
In the autoclave process described above, a uniform pressure is applied atop the preform without use of a rigid top mold, thus making it difficult to achieve good dimensional control.
In both processes, therefore, critical dimensional and process control is required to minimize undesirable defects in the consolidation process for creating accurately dimensioned fan blades. Nevertheless, defective fan blades are still produced and must be rejected, thereby increasing the overall cost of producing acceptable fan blades.
Accordingly, it is desirable to improve consolidation in varying thickness preforms for reducing undesirable defects such as wrinkles, porosity, and delamination.